When gas turbine or turboshaft engines are employed to drive a plant, machinery, or a vehicle, a high numerical reduction ratio is frequently required because of the high output speed (often measured in rotations per minute or rpm) of the turbine. Power transmission of several thousands of horsepower is encountered in many applications. In the case of a stationary plant, or in marine applications, mechanical reliability can be readily achieved if the weight of the gearbox is not critically important. However, with propeller drives for aircraft or rotor drives for helicopters, weight and efficiency of the gearbox are critically important. This requirement led to the widespread adoption of planetary or epicyclic gearboxes in flight applications. Planetary gearboxes achieve their weight advantage over simple gear trains of the same ratio by virtue of increasing the number of mesh points, and hence load-carrying gear engagements, in a given circumferential length of gearing.
With increasing scale and power transmission capacity, the weight of a gearbox increases approximately as a cube function of linear size because the steel elements of the gears span the entire radial distance from the center of rotation to the periphery of the largest gear, usually a ring gear. The tangential force resisting a torque is inversely proportional to the distance from the center of rotation, thus it is clear that while gear tooth loading from tangential force decreases with radius, weight increases disproportionately.
A conventional prior art planetary gear assembly 100 is shown in FIG. 1. Four planet gears 120, 122, 124, 126 orbit and mesh with a central sun gear 130. The planet gears 120, 122, 124, 126 are constrained within and mesh with a ring gear 110.
A prior art planetary reduction gearbox system 200 is shown in FIG. 2, adapted from Dudley's Gear Handbook, McGraw-Hill, 1991. FIG. 2 shows a rotating planetary carrier 254 and a fixed annulus, ring gear 240 having teeth 242. An input shaft 210 rotates as shown by arrow 212 and is connected to a sun gear 214. The sun gear meshes with a plurality of planet gears 220, 230 having teeth 222. The planet gears 220, 230 are on shafts 224, 234 fixed to a planet carrier 250. The planet carrier rotates as shown by arrow 254 and is fixed to an output shaft 252. A double planetary system (not shown) could be formed by combining the gearsets of FIGS. 1 and 2, where the first stage planetary is as gearset 200, but the rotating planet carrier 250 is attached to a sun gear 130 of the second planetary assembly 100. The second planetary assembly 100 usually has a rotating carrier attached to a final output shaft which could be a rotor shaft in a typical helicopter transmission. In the case of a helicopter planetary, the reduction ratio of each planetary reduction is usually limited to 3.875 because that is all that can be achieved with a maximum of six planets revolving around the sun gear.
Such prior art planetary gearbox systems tend to become extremely heavy and thus impractical for aircraft applications when scaled to sizes commensurate with large transport aircraft. Thus, a need remains for a highly efficient, light weight gearbox system for aircraft applications.